MESQUITA M. Eulália., VIEIRA e SILVA J.M., DOMINGUES Hermínia Estação Agronómica Nacional, Quinta do Marquês, 2780 Oeiras, Portugal.

Transcription

1 Scientific registration nº 1892 Symposium nº 25 Presentation: poster Competitive sorption of Cu and Zn by sludge amended acid soils Adsorption compétitive de cuivre et zinc par des sols acides amendés avec des boues MESQUITA M. Eulália., VIEIRA e SILVA J.M., DOMINGUES Hermínia Estação Agronómica Nacional, Quinta do Marquês, 2780 Oeiras, Portugal. 1. Introduction Sewage sludge may be used as an amendment or fertiliser but has serious inconveniences due to the presence of heavy metals. Due to the sewage sludge high content of Cu and Zn it is important to understand Cu-Zn behaviour in adsorptiondesorption reactions (Mesquita et al. 1993). Adsorption-desorption reactions depend on soil and sludge chemical and mineralogical composition and sludge rate and may be represented by Langmuir or Freundlich isotherms. Competitive adsorption has been studied by several researchers. Christensen (1987) established a model for Zn competition with Cd using a simplified Langmuir competitive equation, Murali & Aylmore (1983) applied to multicomponent adsorption models the competitive Langmuir equation as well as Freundlich-type equations. Sheindorf et al. (1981) developed an equilibrium isotherm equation for multispecies adsorption, the Sheindorf-Rehbun-Sheintuch equation (S.R.S.) where all the competing species followed Freundlich-type isotherms, Koopal et al. (1994) applied the Langmuir- Freundlich equation for multicomponent adsorption to heterogeneous surfaces. Working with a very heterogeneous material, such as sewage sludges, we applied the Langmuir-Freundlich equation to study Cu and Zn competitive adsorption-desorption in soils treated with sewage sludge. The Langmuir-Freundlich equation Koopal et al. (1994) extended the Langmuir-Freundlich equation for monocomponent adsorption to competitive adsorption on heterogeneous surfaces. According to the authors this can be achieved by considering component specific non - ideality in the local isotherm function. The Langmuir-Freundlich equation θ i,t =(K i c i ) mi /[(1+(K i c i ) mi ], where θ i, t is the overall coverage of the surface with the adsorbate i, K i is the median value of the adsorbent-adsorbate affinity distribution for i and m i, which has a value between 0 and 1, determines the width of the Sips distribution (Sips, 1948 cit by Koopal et al,1994). For heterogeneous surfaces, considering congruent distribution for the individual components Koopal et al (1994) extended this equation to binary systems, writing the equation as 1

2 θ A A A α = ( K c ) x α β p [( KAcA ) + ( KBc B ) ] α β 1+ [( KAcA ) + ( K Bc B ) ] where α and β are related to the overall exponents for the monocomponent case a and b respectively through p: a=αp and b=βp. Considering a simplified case - congruent distributions for the individual components (local ideality a=b=p and α=β=1 the authors obtained the equation used in this work. A A B B p θ A K A c A x K c K c 1 (2) ( + ) = 1+ ( K c + K c ) A A B. B p 2. Materials and methods. Surface samples from a schistic soil (Eutric Leptsol - soil A) and from a granitic soil (Eutric Cambisol-soil B), (FAO, 1989), were used in this study. The soils were air dried and sieved < 2 mm. The mineralogical composition was determined by X-ray diffraction, using an X-ray diffractometer with Cu radiation and Ni filter. The schistic soil presented in the clay fraction kaolinite > illite> smectite and the granitic soil kaolinite > illite, vermiculite-al» gibsite as the dominant clay minerals. Some soil characteristics are presented on Table 1. Table 1- Soils characteristics. Soils Clay a ph O.M. CEC *Fe 2 O 3 Cu Zn % g kg -1 cmol c kg-1 g kg -1 mg kg -1 mg kg -1 A B soil A - schistic soil; soil B - granitic soil; O.M. - organic matter; CEC - cation-exchange capacity; * Free iron; a ph soil:water - 1:2.5. The soil samples selected were incubated with different amounts of two urban sewage sludges, one from Évora and another from Belmonte. Table 2 present some characteristics of the sewage sludge. 1 p (1) Table 2- Sewage sludge characteristics. ph Cu Zn O.M. Cu- FA Zn- FA Cu-HA Zn-HA mg kg -1 mg kg -1 % % (sludge total content) Évora Belmonte FA (Fulvic acid); HA (Humic acid); Copper and Zn contents were obtained by digestion with HF, HNO 3, HClO 4 and organic matter was fractionated with 0.1 N H 2 SO 4 (free fulvic acid) and 0.1 N NaOH at ph=12 (fulvic and humic acids). In both sludges Cu and Zn were mainly bound to fulvic acids. Copper and Zn adsorption and their distribution in soil+sludge samples were studied after a four months period of incubation at 35ºC. To this effect 0, 1, 2, 4, 6, 8, 10 and 15 g of sludge were added to 125 g triplicate soil samples, well mixed, ground to 200 mesh and submitted to wetting and drying cycles. After incubation the samples were air dried and again ground to 200 mesh. 2

3 Before and after incubation, 3 g subsamples of soil+sludge mixture were treated by 30 ml of 0.5 M Mg(NO 3 ) 2 for 48 h to extract soluble+exchangeable ( exs ) Cu and Zn. After incubation 3 g samples were treated, for 48 h by 30 ml of water to extract water soluble salts. Adsorbed Cu and Zn were determined by sequential extraction. The solutions were centrifuged, decanted and Cu and Zn determined by flame atomic absorption spectrophotometry (AAS). 2.2 Sequential extraction A sequential extraction scheme with specific reagents was used to fractionate Cu and Zn adsorbed by soil+sludge samples (1 g) before and after incubation (Vieira e Silva et al. 1993). The extractions were carried at laboratory temperatures (20±2 C). The samples were washed between extractions with 0.025M Ca(NO 3 ) 2 to remove occluded solutions, decanted and the supernatant discarded. Copper and Zn were determined in the centrifuged solutions of the different fractions by AAS. Adsorbed Cu and Zn obtained by difference between total adsorbed as determined by sequential extraction and soluble+exchangeable Cu and Zn by Mg(NO 3 ) 2 were considered as specifically adsorbed. 3. Results and discussion 3.1 Sequential extraction Copper and Zn distribution in the different soil fractions was determined before and after incubation. The soil fractions considered were: Exchangeable+soluble (Exs), Organically bound ( O.M ), Mn oxide occluded ( MnOx ), Amorphous Fe oxide ( AFeOx ) Crystalline Fe oxide ( CFeOx ). The sum of the amounts present in the different fractions was considered as the value of adsorbed Cu and Zn. Copper and Zn were adsorbed mainly in nonavailable forms, bonded to AFeOx and CFeOx fractions. More Cu than Zn was bound to O.M. fraction. After incubation Cu and Zn bound to Exs and to the more available O.M. fractions (F.A.) decreased and Cu bound to less available O.M. fractions increased. The organic fraction seems to be a source of specific Cu sorption sites (McLaren & Crawford, 1973) but organic complexation could lead to increased mobility in the soil since Cu and Zn form stable complexes with fulvic acid (Kabata-Pendias &Pendias, 1984, cit Harter, 1991). The sewage sludge from Belmonte, added to the granitic soil, contained more Zn and less Cu than the sludge from Évora added to the schistic soil (Table 2), which accounts for the different content of these metals present in either soil. 3

4 3.2 Adsorption - desorption reactions Data from desorption reactions is presented on Tables 3 & 4. Table 3 - Schistic soil. Analytical data from soil+sludge samples Sample ph Cu (mg L -1 ) Cu (mg kg -1 ) Zn (mg L -1 ) Zn (mg kg -1 ) solution soil solution soil n * exs a sol b exs b exc b spc b tot exs a sol b exs b exc b spc b tot n -g of sludge added to 125 g of soil sample; * ph of the Mg(NO 3 ) 2 equilibrium solution a- Metal conc. before incubation; b- Metal conc. after incubation; exs - soluble+exchangeable; exc.-exchangeable; spcspecifically adsorbed; tot- of 9metal extracted by sequential extraction. Due to the composition of the sewage sludges, more Cu was present in the Mg(NO 3 ) 2 extract from the schistic soil and more Zn in the granitic soil extract. In both soils, however Cu and Zn were mainly specifically adsorbed. Table 4 - Granitic soil. Analytical data from soil+sludge samples Sample ph Cu (mg L -1 ) Cu (mg kg -1 ) Zn (mg L -1 ) Zn (mg kg -1 ) solution soil solution soil n * exs a sol b exs b exc b spc b tot exs a sol b exs b exc b spc b tot n -g of sludge added to 125 g of soil sample; *ph of the Mg(NO 3 ) 2 equilibrium solution a Metal conc. before incubation; b- Metal conc. after incubation; exc.-exchangeable; exs-soluble+exchangeable; spc - specifically adsorbed; tot - of metal extracted by sequential extraction Exchangeable+soluble ( Exs ) Cu and Zn decreased (Fig 1) being adsorbed in non available forms. However, Sauerbeck and Styperek (1986) cit. Smith (1996), suggested that no generalizations could be drawn about the potential reduction in metal availability as a function of time. 4

5 "Exs"Cu and Zn decrease Added Sludge (g/125g soil) Cu(A) Zn(A) Cu(B) Zn(B) Fig 1 Decrease in Exs Cu and Zn after incubation. A-Schistic soil; B-granitic soil. The decrease in Cu, in the schistic soil was slightly larger than the decrease in Zn. In the granitic soil, as the sludge applied had a very high Zn content the decrease in Exs Zn was much larger than Cu decrease. In both soil these variations were statistical significant according to Student s test. Desorption data fitted power equations of the Freundlich type q=kc n where k and n are equation constants (Table 5). Table 5- Freundlich equations constants. Schistic soil Granitic soil k n r 2 k n r 2 * Cu exc * Cu exc * Zn exc *Zn exc # Cu spc # Cu spc # Zn spc # Zn spc * Exchangeable vs water soluble; # specifically adsorbed vs water soluble+exchangeable. 3.3 Copper and Zinc interaction Distrbution coefficients The distribution coefficients K d =q/c, where q is the amount of adsorbed cation and C the cation concentration in the equilibrium solution, gives a measure of cation interaction and of their relative mobility. A large K d value represents a high soil affinity with the metal and indicates a low mobility and a low K d value indicates a high mobility through the soil profile (Christensen 1989). As adsorption is represented by Freundlich isotherms the distribution coefficient may be written K M d =kc (n-1). The values of both K Cu Zn d and K d decreased when cation concentration in solution increased, Cu and Zn mobility increasing with the amount of sludge added to the soil. In the schistic soil exchangeable Zn was more affected by Cu than exchangeable Cu by Zn. Specifically adsorbed Cu and Zn, were only slightly affected by the other cation as Cu Zn K d spc and K d spc present a less pronounced decrease with increasing sludge rates 5

6 Table 6 - Values of distribution coefficients calculated from the isotherm s equations. Schistic soil Granitic soil Sample Kd Cu exc Kd Zn exc Kd Cu spc Kd Zn spc Kd Cu exc Kd Zn exc Kd Cu spc Kd Zn spc In the granitic soil exchangeable and specifically adsorbed Cu were more affected by Zn than Zn by Cu, as can be observed on table 6. The different behaviour of the schistic and granitic soil concerning exchangeable Cu and Zn seems dependent sewage sludge content of Cu and Zn. On the other hand, specifically adsorbed Cu and Zn behaviour seems more dependent on the heavy metal Competitive Langmuir Freundlich equation The parameters of the Langmuir Freundlich equation (equation 2) were calculated applying multilinear regression analysis to desorption data. The equation parameters are presented on Table7. Table 7 - Langmuir Freundlich equation parameters schistic soil (A) granitic soil (B) Q m k Cu k Zn p r 2 Q m k Cu k Zn p r 2 Cu exc Zn exc Cu spc Zn spc If p 1 the Langmuir-Freundlich equation can be written as q A =Q A K A c A /(1+K A c A+ K B c B ), and the competition of cation B for adsorption dependent of K B c B. The calculated parameters gave p 1 for specifically adsorbed Zn in the schistic soil and specifically adsorbed Cu and Zn in the granitic soil, confirming the small variation of specifically adsorbed Cu and Zn with increasing sludge rates as indicated by distribution coefficients values. 4 Conclusions After a four months incubation, exchangeable Cu and Zn decreased slightly in the soil+sludge samples considered in this study. Data from sequential extraction evaluated Cu and Zn distribution in the soil+sludge samples, showing that only small amounts of these metals were in available forms, Cu and Zn being mainly specifically adsorbed. The analytical data from adsorption-desorption reactions fitted power equations of the Freundlich type. Distribution coefficients decreased as sludge rates increases increasing Cu and Zn mobility. The small variation of specifically adsorbed Cu and Zn as indicated by distribution coefficients values is confirmed by the parameters of the Langmuir-Freundlich equations.. The results of this study showed that as Cu and Zn are mainly present in nonavailable forms and considering distribution coefficients values Cu and Zn will only 6

7 present an environmental risk if high sludge rates were used as mobility increased with sludge rates. References Christensen. T.H., Cadmium soil sorption at low concentrations: VI. A model for zinc competition. Water, Air and Soil Pollution, 34: FAO - UNESCO, Carte mondiale des sols. Légende révisée. Rapport sur les ressources en sols du monde 60. Organisation des Nations Unies pour l'alimentation et l'agriculture Harter, R. D., (1991). Micronutrient adsorption-desorption reactions in soil. pp In Micronutrients in Agriculture, Ed Mortvedt, J.J., Cox, F. R. Shuman, L. M. and Welch, R. M. 2nd ed. (Soil Sci. Soc. Am. Book series, nº4). Madison, Wisconsin, USA Koopal, L. K., van Riemsdijk, W. H., de Witt, J. C. M. and Benedetti, M. F, Analytical isotherm equations for multicomponent adsorption to heterogeneous surfaces. J. Colloid Interface Sci. 166: McLaren, R. G. and Crawford, D. V., Studies on soil copper (II): The specific adsortion of copper by soils. J. of Soil Sci. 24: Mesquita, M.E., Vieira e Silva, J.M., Sequeira E.M. and. Domingues, H Copper and zinc sorption and interaction on a schistic soil and two sewage sludges. Environmental Pollution - icep.2, vol European Centre for Pollution Research, London. Murali, V. and Aylmore, L. A. G., (1983) Competitive adsorption during solute transport in soils: 1. Mathematical models. Soil Science. 135: Sheindorf, C. H., Rehbun, MM. And Sheintuch, M. (1981). A Freundlich type multicomponent isotherm. J Colloid Interface Sci. 79: Smith, S. R., Agricultural recycling of sewage sludge and the environment. CAB International, Wallingford, U. K. Vieira e Silva, J.M., Domingues, H. and Mesquita, M.E., Sequential extraction of copper and zinc from sewage sludges. Use of organic solvents. Int. J. Environ. Anal. Chem.51; Keywords : sewage sludge, competitive adsorption, Copper, Zinc, distribution coefficients Mots clés : boues d épandage, adsorption,compétitive, cuivre, zinc, coefficient de distribution 7